The dramatic progress of the computer technology over the last four decades has been fueled by an unparalleled development of the basic hardware elements: storage devices, processors and displays. In all three areas, the art and science of surface structuring is of essential importance. Technology trends towards miniaturization and integration, as well as towards improved performance, reliability and productivity require increasingly better control of surfaces and interfaces down to the molecular or atomic level.
Recently, research directed to the controlled fabrication of small structures has been greatly inspired by the success of scanning probe microscopes (hereafter, “SPM”). Numerous examples confirm that the pointed probe tip of the SPM is not only able to monitor variations in the sample surface structure with atomic or near-atomic resolution, but it also can be used to modify the surface on a similar atomic or molecular scale. It was demonstrated by T. A. Jung, R. R. Schlittler, J. K. Gimzewski, H. Tang, and C. Joachim in Science, Vol. 271, p. 181, 1996, for instance, that individual molecules can be moved into prescribed fixed new positions and/or be modified without change of position under the influence of the SPM tip. In the pursuit of such investigations, it was found that molecular flexibility and diffusibility plays an important role for such molecular repositionings.
Fixed ordered molecular layers on a substrate are generated, e.g. by molecules forming Langmuir-Blodgett (LB) films, or self-assembled monolayer (SAM) films, or by undergoing cooperative self-assembly, or by being deposited by sublimation or by molecular beam epitaxy. The assembly within the molecular overlayer is driven by molecule-substrate interaction, as well as covalent or non-covalent intermolecular forces. Depending on the roughness of the atomic surface potential, at a given temperature, single molecules can diffuse over the substrate or are immobilized. Within a grown self-organized molecular superstructure, the individual molecules cannot move due to local forces and are fixed by their local environment.
One of the most used storage devices at the moment is the hard-disc, which uses magnetic units to store information. Single bits are realized by a certain area of magnetizable material. The magnetization of these bits are changed and read out by a head. The bits are arranged in tracks on a disc, which is spinning fast around its axis. Different patents and prior art references exist that cover the idea of writing in a “phase change medium” by applying voltage pulses. However, usually bulk material is used to achieve these methods and the mechanism of changing the structure is often induced by resistive heat. Also, erasing often is performed by heating as described in the following examples:
“Information recording and reproducing” describes a phase change material, wherein change of the phase is obtained by the resistive heat of the flowing current (by applying a voltage) as disclosed in EP0665541 A2 and U.S. Pat. No. 6,101,164.
“Phase change media for ultra-high-density data-storage devices” describes two solid phases, wherein transition between the phases occurs by heating the bit storage region as disclosed in EP1233418 A1.
“Non-volatile memory device” describes a liquid crystal that is heated by a pair of electrodes, thereby changing the phase as disclosed in U.S. Pat. No. 5,444,651.
Liquid crystals are a form of matter that lie in-between disordered liquids and ordered crystals. They are created mostly using long stretched molecules of about 10 to 100 atoms, which have one or more benzene rings or double bonds in their center. Liquid crystals were invented more than a 100 years ago, but were investigated more precisely only the last three decades when their technical usage grew important. Today, more than 10,000 types of molecules are known for building such liquid crystals. The molecules of the liquid crystals can arrange in several different sterical configurations; and, although these patterns look rather crystalline, they have typical properties of a liquid. For instance, the viscosity of liquid crystals is on the order of 0.01 kg m−1 s−1, with a vanishing elastic modulus and a rate of change of spatial location (i.e., “hopping rate”) of 10−7 s. The molecules can be shifted relatively easily between each other, but keep their parallel spatial arrangement.
A Liquid-Crystal Display (hereafter, “LCD”) mainly contains a 50-100 μm thick layer of a nematic liquid between two crossed optical polarization filters. The orientation of the molecules at the contact area (e.g., boundary surface orientation, or “Grenzflächenorientierung”) is turned 90° with respect to each other and is equivalent to the direction of the light polarization of the filters. Without an electrical field, light can travel through the device. With an applied electrical field, the molecules in the center of the device are turned into the direction of the field. The light meets the second polarization filter without a rotation of the direction of the polarization and therefore is adsorbed there.
However, the prior art liquid crystal devices have the following drawbacks. First, they require a three-dimensional structural transition that is a relatively bulky, multilayer structure that is about 50-100 μm thick. Second, the device needs to have fixed top and bottom electrodes in order to provide an electric field and these electrodes must be selected based upon the properties at the interface between the liquid crystal and the electrode. Third, the switching effect achieved by liquid crystals is based on a rotation of individual molecules that are located at a fixed position, although the molecules have a high rate of change of spatial location of 10−7 s, which is also referred to as the “hopping rate.”
“Information recording medium and method for manufacturing the same” describes a reversible phase change between electrically or optically detectable states that is caused by electric energy, or electromagnetic energy, in lattice defects (at least a part of the lattice defect has to be filled with an element other than an element constituting the crystal structure) as disclosed in EP 1170147 A1.
“Systems and methods for providing a storage medium” describes cholesteric liquid crystals having a plurality of display layers wherein texture is changed by applying voltages as disclosed in U.S. Pat. No. 6,392,725 B1. However, this patent is another example of a three-dimensional device.
Other approaches that use a bulk 3-dimensional dipolar material to store data are disclosed in “Reversible, Nanometer-Scale conductance transitions in an organic complex,” which describes molecules with a permanent dipole moment forming a 3-dimensional thin film (approx. 20 nm thick), that has a local reversible structural transition induced by an electric field with phase transition between ordered-disordered states as disclosed in Gao et al., Phys. Rev. Lett. 84 (2000) 1780.